The invention relates to a test that can be used to predict whether cognitively competent individuals are likely to develop Alzheimer""s disease. More specifically, the invention relates to measurement of the ratio of amyloid protein xcex2 (Axcex2)40 to Axcex242 in cerebrospinal fluid lipoprotein fractions as a predictor of this condition.
Alzheimer""s disease (AD) is a highly troubling cause of dementia, especially in elderly people. Confirmation of the presence of AD is generally done post-mortem and the disease is characterized by neurofibrillary tangles and neuritic plaques. The diagnosis even of existing AD in patients is not always accurate and can only be confirmed by post-mortem analysis. Early diagnosis prior to the onset of dementia is essentially nonexistent although there are predictors of the disease which could most readily be characterized as risk factors. Among these risk factors is the presence of the E4 allele of apolipoprotein E (ApoE). In humans, ApoE has three isoforms (ApoE2, ApoE3 and ApoE4) that differ by only a single amino acid substitution. E4 (Arg112, Arg158) and E2 (Cys112, Cys158) are less common than E3 (Cys112, Arg158). ApoE is a major apolipoprotein constituent of cerebrospinal fluid (CSF) where it is associated with high density lipoprotein (HDL)-like particles. The ApoE in CSF is derived from the brain. Thus, an aberration in the ApoE component is logically related to neurological disease.
Growdon, J.H. in Arch Neurol (1999) 56:281-283 summarizes the status of antemortem diagnosis of AD. As reported, a working group convened in 1997 to evaluate this resulted in a consensus statement entitled xe2x80x9cMolecular and Biochemical Markers of ADxe2x80x9d which appeared in Neurobiol Aging (1998) 19:109-116. This study observed that no clinical biomarker has achieved universal acceptance.
Among the markers currently under consideration are those related to the proteins which account for the features found in Alzheimer brains postmortem. The neurofibrillary tangle is composed primarily of a hyperphosphorylated tau protein, a cytoskeletal protein. The neuritic plaque contains a core of amyloid protein, much of which is a 42-amino acid peptide (Axcex242) derived from proteolytic cleavage of a larger precursor protein. Another form of this protein derived from the same precursor contains only 40 amino acids (Axcex240). Blood tests based on these proteins do not seem to correlate well with AD.
Others have measured levels of these proteins in CSF. It appears that elevated levels of tau are present in CSF from AD patients (Clark, C.M., et al., The Dementias, Butterworth-Heinemann, Boston, Mass. (1998) 285-304). It has been shown that individuals with AD have decreased Axcex242 levels in their cerebrospinal fluid (Galasko, D., et al, Arch. Neurol. (1998) 55:537-545; Ida, N., et al., J. Biol. Chem. (1996) 271:22908-22914; Kanai, M., et al., Japan. Ann. Neurol. (1998) 44:17-26; Motter, R., et al., Ann. Neurol. (1995) 38:643-648; Tamaoka, A., et al., J. Nezirol. Sci. (1997) 1997:41-45. The Motter paper indicated Axcex242 levels in AD CSF were independent of ApoE genotype, but the larger study by Galasko showed an inverse relationship between the E4 allele and Axcex242 levels. The Kanai paper reported an increase in Axcex240/Axcex242 levels in CSF from AD patients, but interpolation from regression analysis suggested that this increase begins prior to the onset of clinical symptoms. It has also been found that in AD patients, Axcex242 levels in CSF are decreased, whereas the levels of total Axcex2 proteins (Axcex242+Axcex240) are substantially the same in AD patients and in normal controls (Motter, et al., Supra). The Growdon article (supra) concludes, however, that these alterations in tau and Axcex242 do not occur with sufficient frequency and magnitude so that they offer diagnostic value.
Thus, at present, there appears to be no satisfactory diagnostic marker even for existing AD, much less a diagnostic predictor for an individual, who although exhibiting normal cognitive responses, will inevitably, or most likely, develop AD. The test described herein meets the need for such a diagnostic. Details of this test are described in an article by Fagan, A.M., et al., Ann. Neurol. (2000) 48:201-210, incorporated herein by reference.
The present invention offers a diagnostic method which identifies those individuals who will later in life be at higher risk for developing AD. The diagnostic is based on the surprising discovery that the lipoprotein fraction of CSF in such individuals has increased ratios of Axcex240 to Axcex242. Thus, the invention offers a relatively noninvasive and straightforward method to identify those individuals at high risk for subsequent development of AD. Although the ratio in the lipoprotein fraction is the most highly correlated with the probability of the onset of AD, the value of the ratio correlates strongly with the ratio in total CSF. Thus, although less reliable, total CSF could also be used as the substrate for this ratio determination.
Accordingly, in one aspect, the invention is directed to a method to identify an individual having an enhanced risk for development of AD, which method comprises determining the ratio of Axcex240 to Axcex242 preferably in the lipoprotein fraction of the CSF in said individual and comparing this ratio to the corresponding ratio in the population as a whole or to the ratio found in the CSF lipoprotein of a subgroup of the population not at risk for AD. If the subject is found to have an elevated ratio compared to these populations, that subject is at increased risk for developing Alzheimer""s disease.
In other aspects, the invention is directed to kits containing suitable reagents for measurement of the Axcex240/Axcex242 ratio.